Lets consider the following code (compiles successfully with clang++ 7.0.0, compiler arguments are -std=c++17 -Wall -Wextra -Werror -pedantic-errors
):
[temp.names](Names of template specializations)/1:
A template specialization can be referred to by a template-id:
simple-template-id: template-name < template-argument-listₒₚₜ > template-id: simple-template-id operator-function-id < template-argument-listₒₚₜ > literal-operator-id < template-argument-listₒₚₜ > template-name: identifier
As you can see, there is no conversion-function-id
conversion-function-id:
operator conversion-type-id
mentioned in the template-id grammar.